The invention relates to an arrangement for measuring the distance separating the arrangement from a moving body. The arrangement comprises transmission means for transmitting a wave in a measuring field, receiving means for receiving the wave reflected from the moving body moving in the measuring field and also for supplying a beat signal at the Doppler frequency, delimiting means for providing in the measuring field at least two datum lines, and processing means for processing the distance data obtained from the Doppler frequency measurement when the moving body crosses the datum lines.
German patent applications Nos. 2,315,815, 2,315,816 and 2,365,765 disclosed such an arrangement. That arrangement is installed on a moving body and is used in a control for keeping the moving body at a fixed distance from a rail by means of electromagnetic suspension. The distance measuremeent is derived from the Doppler frequencies at instants defined by datum lines obtained with the aid of a laser beam transmitted from the moving body and reflecting from reflectors disposed equidistantly along the rail.
This arrangement requires the presence of elements (the reflectors) arranged opposite each other. There are certain uses for which the presence of these facing elements is not tolerated. That is the, for example, when the arrangement is used for triggering a mine when a nearby vehicle passes the mine. It is then important for the arrangement to be unobtrusive, which excludes the presence of external objects for cooperation with the arrangement. It should be noted that, for that type of usage, the distance measurement is indispensable so as to ensure that the mine is not triggered until the vehicle enters its effective range.